1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine or the like, and to a method of manufacturing the same.
2. Description of the Related Art
A spark plug for use in a combustion apparatus, such as an internal combustion engine, includes, for example, a center electrode extending in an axial direction; an insulator surrounding the center electrode; a cylindrical metallic shell externally assembled to the insulator; and a ground electrode whose proximal end portion is joined to a front end portion of the metallic shell. The ground electrode is bent at its substantially intermediate portion so that a distal end portion of the ground electrode faces a front end portion of the center electrode, thereby forming a spark discharge gap between the front end portion of the center electrode and the distal end portion of the ground electrode.
In recent years, a technique has been known for improving erosion resistance by means of joining a noble metal tip to a distal end portion of the ground electrode in a region adapted to form the spark discharge gap. According to a generally employed method for joining the noble metal tip to the ground electrode, the periphery of a contact surface between the ground electrode and the noble metal tip is irradiated with a laser beam. A fusion zone is thereby formed where metal materials of the ground electrode and the noble metal tip are fused and joined together (refer to, for example, Patent Document 1).
Meanwhile, from the viewpoint of further improving erosion resistance, increasing the diameter of the noble metal tip has been considered for increasing the area of a surface (discharge surface) adapted to form the spark discharge gap of the noble metal tip.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2005-158323
3. Problems to be Solved by the Invention
However, thermal stress difference becomes relatively large between the ground electrode and the noble metal tip having an increased diameter. Also, since the noble metal tip has an increased diameter, when the noble metal tip is joined to the ground electrode, the penetration depth of the fusion zone may become excessively small as compared with the size of the noble metal tip. That is, in the case of using a noble metal tip having an increased diameter, the thermal stress difference between the ground electrode and the noble metal tip increases. As a result, it becomes difficult to form the fusion zone to a sufficient depth, the fusion zone having the role of absorbing the thermal stress difference. Therefore, there is deep concern about the progress of oxide scale in a joined portion between the ground electrode and the noble metal tip and associated separation of the noble metal tip from the ground electrode.
In this connection, increasing the irradiation energy of a laser beam may be contemplated for increasing the penetration depth of the fusion zone. However, a mere increase in irradiation energy may cause the fusion zone to reach or come close to the discharge surface, potentially resulting in a failure to sufficiently exhibit the effect of improving erosion resistance by providing the noble metal tip.
Another means for preventing the fusion zone from reaching or coming close to the discharge surface by increasing the thickness of the noble metal tip might also be contemplated. However, since a noble metal alloy used to form the noble metal tip is expensive, if the noble metal tip that is increased in diameter is further increased in thickness, the cost may increase considerably. In actuality, reducing the thickness of the noble metal tip is rather desired for restraining, to the greatest possible extent, an increase in cost associated with an increase in diameter.